550 



SCIENCE PROGRESS 



graphs illustrating the transformation of the unstable into the 

 stable form of 2 : 4-dibromoacetanilide is given in Plate II. 



If small quantities of the needle-shaped form of either com- 

 pound are moistened with alcohol, sealed up in a tube and 

 plunged into a heated oil-bath, transformation in each case is 

 complete in a few minutes at ioo° and just below the melting- 

 points of the compounds is so rapid as to appear instantaneous. 



The melting-points obtained, therefore, are those of the 

 stable forms, since in consequence of the rapid change the 

 unstable forms completely disappear before fusion occurs. 



Although transformation of these unstable forms occurs so 

 easily, it has been found possible to shake them sufficiently long 

 with alcohol at moderate temperatures to obtain saturated 

 solutions and hence to determine their solubilities. 



The results show excellently how solubility depends upon 

 the exact nature of the solid present and afford a good illustra- 

 tion of van't Hoff's generalisation that the stable modification 

 of any pair of polymorphs must have the smaller solubility. 



The following tables give the solubilities of the two forms of 

 />-bromoacetanilide and of 2 : 4-dibromoacetanilide in ethyl 

 alcohol, the numbers recording the weight of anilide in grams 

 in 100 grams of solution. 



The solubilities were determined by shaking in a constant- 

 temperature bath excess of the anilides with the solvent for a 

 number of hours sufficient to ensure saturation. A sufficient 

 quantity of the saturated solution was then taken out, weighed, 

 and the solvent removed by evaporation . The weights of solute 

 and solvent were thus obtained. 



Between o° and the melting-point the compact form both of 



